FIG. 1 shows a conventional electromagnetically-fixable electric drill unit. The electromagnetically-fixable electric drill unit has an electromagnetic base 2 at the bottom of a support frame 1 thereof. The drill unit is mounted on a workpiece by an attracting force of the electromagnetic base. The support frame 1 supports an electric drill 4 so that the electric drill is vertically moved by rotating a handle 5. The electric drill 4 has a drill motor 6 and an arbor 8 rotatably driven by the motor, and an annular cutter 10 is secured to the arbor. The electric drill 4 is provided with a tank 12 for storing cutting oil to be supplied to the annular cutter 10.
FIG. 2 is a plan view of the electric drill unit. Here, the handle 5, the tank 12 and some other elements are omitted. As shown, the electric drill 4 is adapted to be guided by means of a dovetail-shaped linear guide mechanism 11 so as to move vertically with respect to the support frame 1. Specifically, the electric drill 4 is provided with a slide plate 19 having a sliding portion 13 which slidably engages with a dovetail-shaped guide groove 15 of a guide secured to the support frame 1.
However, this type of guide mechanism has a tendency to become difficult to properly guide the electric drill 4 in the vertical direction due to looseness caused by wear thereof, thereby hindering cutting operation.
Therefore, conventionally, the guide is provided with threaded holes arranged at appropriate intervals in the longitudinal direction of the guide and extending from a side surface of the guide to the guide groove 15, and adjusting screws 16 (see FIG. 1) are threadably engaged with the threaded holes. Thus, if looseness is generated, these adjusting screw members are tightened to eliminate the looseness. Alternatively, the support frame 1 is provided with a bracket 14 for rotatably holding the arbor 8. Thus, even if looseness is generated in the guide mechanism 11, the arbor 8 and the annular cutter 10 attached thereto can be prevented from wobbling (see Japanese Patent No. 2863087).
In a case in which the bracket 14 is provided for preventing wobbling of the arbor, however, an oil supply pipe 17 extending from the tank 12 to supply cutting oil to the annular cutter 10 has to be connected to the upper end of the arbor 8 (see FIGS. 1 and 3). In this case, the distance from the upper end of the arbor where the cutting oil is supplied to cutting teeth at the distal end of the annular cutter 10 is large. As a result, the cutting oil cannot be adequately supplied to the cutting teeth when the electric drill unit is used in a tilted manner in order to drill a hole in a wall or the like.